fix(builder): exec the runner
fix(builder): Non-root slugrunner and slugbuilder

This improves the compatibility to heroku and stops running apps as root
fix(builder): Reverted to default php buildpack

The pervious compatibility fixes made the heroku php build pack work
out-of-the-box again
Merge pull request #1972 from johanneswuerbach/buildpack-env

fix(builder): Non-root slugrunner and slugbuilder